Passive acoustical filters with filled expansion chamber

ABSTRACT

A passive acoustical filter includes a first opening to receive ambient sound, a second opening to output filtered sound, and a first low pass filter element coupled between the first opening and the second opening. The first low pass filter element includes a first expansion chamber filled with a material in which a speed of sound is lower than a speed of sound in air.

RELATED APPLICATION INFORMATION

This patent is a continuation of patent application Ser. No. 14/601,926,entitled PASSIVE ACOUSTICAL FILTERS INCORPORATING INSERTS THAT REDUCETHE SPEED OF SOUND, filed Jan. 21, 2015, which claims priority fromProvisional Patent Application No. 61/929,788, entitled PASSIVE AUDIOEAR FILTERS, filed Jan. 21, 2014, both incorporated herein by reference.

This patent is related to patent application Ser. No. 14/601,901,entitled PASSIVE AUDIO EAR FILTERS WITH MULTIPLE FILTER ELEMENTS, filedJan. 21, 2015, which also claims priority from Provisional PatentApplication No. 61/929,788.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND

1. Field

This disclosure relates generally to passive audio ear filters that canattenuate certain sound frequencies and allow other frequencies to passthrough unchanged.

2. Description of the Related Art

Exposure to sound at certain sound pressure levels and sound frequenciescan, over time, cause hearing loss.

Humans' perception to sound varies with both frequency and soundpressure level (SPL). For example, humans do not perceive low and highfrequency sounds as well as they perceive midrange frequencies sounds(e.g., 500 Hz to 6,000 Hz). Further, human hearing is more responsive tosound at high frequencies compared to low frequencies. FIG. 1illustrates equal loudness contours defined in ISO (InternationalStandards Organization) Standard 226(2003). The X axis represents soundfrequency measured in Hertz (Hz) and the Y axis represents soundpressure level measured in decibels (dB) relative to a pressure level of2×10⁻⁵ Pascal. The unit of measurement for loudness levels is the phon,and is arrived at by reference to equal-loudness contours. FIG. 1 showsequal loudness contours for loudness levels of 20, 40, 60, 80, and 100phon. Each equal-loudness contour defines the sound pressure level, overthe frequency spectrum, for which a listener perceives a constantloudness when presented with pure steady tones. FIG. 1 also shows thehearing threshold level.

Hearing protection that attenuates sound equally at all frequencies, orotherwise without regard to the variation of hearing sensitivity withfrequency, may attenuate potentially damaging sounds at the expense ofpleasurable sounds. For example, an ear filter providing uniformattenuation of 20 dB would reduce loudness by about 20 phon at 1 kHz and40 phon at 20 Hz. Thus the relative loudness of low frequency soundswould be substantially reduced relative to the loudness of higherfrequency sounds. However, with attention to the hearing responsecurves, it is possible to design ear filters that attenuate damagingsound levels and maintain, or even enhance, desired sounds.

There are many situations where people desire protection from sounds atcertain frequencies, while allowing sounds at other frequencies to reachtheir ears. For example, at a concert, concert goers might want to enjoythe music, but also be protected from the mid-range and high levels ofsound frequencies that cause damage to a person's hearing. On anairplane, passengers might wish to block out the roar of the engine, butnot conversation. At a sports event, fans might desire to hear theaction of the game, but receive protection from the roar of the crowd.While sleeping, people might want protection from all auditorydisturbances. These are just a few common examples where people wish tohear some, but not all, of the sound frequencies in their environment.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of equal-loudness contours.

FIG. 2 is a schematic cross-sectional view of an exemplary low-passfilter element.

FIG. 3 is a schematic cross-sectional view of an exemplary high-passfilter element.

FIG. 4 is a schematic cross-sectional view of an exemplary band-rejectfilter element.

FIG. 5 is a schematic diagram of a passive acoustical filter including aseries of low-pass filter elements in parallel with a high-pass filterelement.

FIG. 6 is a graph of attenuation curves relating to the passiveacoustical filter of FIG. 5.

FIG. 7 is a schematic diagram of an exemplary high-pass filter element.

FIG. 8 is a graph of attenuation curves for two low-pass filter elementssuch as the low-pass filter element shown in FIG. 7.

FIG. 9 is a schematic diagram of a passive acoustical filter including alow-pass filter element in parallel with a high-pass filter element.

FIG. 10 is a graph of attenuation curves relating to the passiveacoustical filter of FIG. 9.

FIG. 11 is a schematic diagram of a passive acoustical filter includingtwo low-pass filter elements in series.

FIG. 12 is a graph of attenuation curves relating to the passiveacoustical filter of FIG. 11.

FIG. 13A is a graphical image of components of a representative musicacoustical filter.

FIG. 13B is a schematic cross-sectional view of a music acousticalfilter assembled form the components shown in FIG. 13A.

FIG. 14 is a schematic diagram of the music acoustical filter assembledform the components shown in FIG. 13A.

FIG. 15 is a graphical image of the passive acoustical filter assembledfrom the components shown in FIG. 13A.

FIG. 16 is an exploded graphical image of another music acousticalfilter.

FIG. 17 is a graphical image of the music acoustical filter assembledfrom the components shown in FIG. 16.

FIG. 18 is a graphical image of an exemplary music acoustical filterwithin an earbud.

DETAILED DESCRIPTION

Passive acoustical filters incorporating multiple filter elements canprotect the ear from damaging sound frequencies while allowing lessdamaging sound frequencies to reach the ear. In this document, the term“filter element” refers to an acoustic filter that provides a singlefilter function such as a low-pass, high-pass, band-pass, or band-rejectfilter function. The term “passive acoustical filter” refers to a filterdevice that includes one or more filter element coupled between anambient and a listener's ear. Two or more filter elements are consideredto be “coupled between the ambient and the listener's ear” if ambientsound must pass through at least one of the filter elements beforereaching the listener's ear. Two or more filter elements are consideredto be “in series” if ambient sound must pass consecutively through allof the two or more filter elements before reaching the listener's ear.Two or more filter elements are considered to be “in parallel” if thefilter elements provide alternate paths for ambient sound to reach thelistener's ear. Unless otherwise stated, ambient sound may be dividedapproximately equally between parallel filter elements. The term“earbud” means an apparatus configured to fit, at least partially,within and be supported by a user's ear. Typically, a portion of anearbud fits against or within the user's outer ear canal. Other portionsof an earbud may fit within the concha or pinna of the user's ear.

Such passive acoustical filters can be designed compactly in order tofit within an earbud, headphone, or other apparatus that can be placedinto or outside an ear. Further, such passive acoustical filters can bedesigned to attenuate certain damaging and/or disturbing soundfrequencies associated with specific environments and/or activities. Forexample, a passive acoustical filter may include a series of low-passfilter elements in parallel with a high-pass filter element and canattenuate damaging mid-range frequencies associated with attending aconcert. For further example, a passive acoustical filter may provide alow-pass filter element and a high-pass filter element in parallel, andcan attenuate mid-range frequencies associated with attending a sportsevent or participating in motor sport activities. Yet another exemplarypassive acoustical filter may include a series of low-pass filterelements, and can attenuate mid and high-range frequencies associatedwith sounds disturbing to sleep. Other passive acoustical filters arepossible that provide different types of filter elements alone, inparallel, in series, and/or in parallel/series combinations to attenuateunwanted frequencies associated with specific activities and/orenvironments, while allowing desired frequencies to pass through.

FIG. 2 is a schematic cross-sectional view of an exemplary low-passfilter element. The low-pass filter element 200 includes a main branch210 and a single expansion chamber 220. The expansion chamber 220 may befilled with an insert 225. In operation, ambient sound can enter anopening 230 of the low-pass filter 200, and certain sound frequenciescan be filtered as the ambient sound passes through the main branch 210and expansion chamber 220. The resulting filtered sound can exit theopening 240 at the opposite end of the low-pass filter element 200.

The following equation can be used to calculate the dimensions neededfor a low-pass filter element to achieve a desired cutoff frequency(i.e., the frequency at which the filter starts to have effect):

$f_{c} = \left( \frac{cS}{\pi \; {L\left( {S_{1} - S} \right)}} \right)$

-   -   Where:    -   fc=the cutoff frequency of the low-pass filter element 200;    -   c=speed of sound within the expansion chamber 220;    -   S=diameter of the main branch 210;    -   S₁=diameter of the expansion chamber 220; and    -   L=length of the expansion chamber 220.

The cutoff frequency of the low pass filter element 200 is dictated bythe shape and size of the filter element, as well as the medium withinexpansion chamber of the filter element. The conduction of sound wavesthrough a medium is dependent upon the ratio of the bulk modulus of themedium to the density of the medium, and is governed by theNewton-Laplace equation:

$c = \sqrt{\frac{K}{\rho}}$

-   -   Where:    -   c is the speed of sound;    -   K is the bulk modulus of the medium; and    -   ρ is the density of the medium.

The speed of sound in air is 343 meters per second (at 20° C., 1 atm).In some applications, the size of a filter element having a desiredcutoff frequency can be reduced by filling the expansion chamber with aninsert made of a material having a ratio of bulk modulus to density(K/ρ) less than that of air. Filing the expansion chamber with such amaterial will slow down the speed of sound. For example, in the case ofa low-pass filter element, filling the expansion chamber with a materialthat lowers the speed of sound by a factor of two can reduce the lengthof the expansion chamber equally by a factor of two. A reticulatedmaterial that has a K/ρ ratio lower than air may be placed inside theacoustical filters to reduce the speed of sound, while still allowingpassage of sound through the filter. In this context, “reticulated”means forming or formed like a network or a web. Suitable reticulatedmaterials may include open-cell or closed-cell foams made ofpolyurethane, polyester, polystyrene, or other plastic. Other suitablereticulated materials include organic fibers like cotton, bamboo, andyarn.

FIG. 3 is a schematic cross-sectional view of an exemplary high-passfilter element. The high-pass filter element 300 includes a main branch310 and a side branch 320. The side branch may optionally be filled withan insert 325. In operation, ambient sound can enter an opening 330 ofthe high-pass filter element 300, and certain sound frequencies can befiltered as the ambient sound passes through the main branch 310 andside branch 320. The resulting filtered sound can exit the opening 340at the opposite end of the high-pass filter element 300.

The following equation can be used to calculate the dimensions neededfor a high-pass filter element to achieve a desired cutoff frequency:

$f_{c} = \left( \frac{c\; a^{2}}{2{SL}} \right)$

-   -   Where:    -   fc=the cutoff frequency of the high pass filter element 300;    -   c=speed of sound in the insert 325;    -   S=diameter of the main branch 310;    -   L=the length of the side branch 320; and    -   a=radius of the side branch 320.

FIG. 4 is a schematic cross-sectional view of an exemplary band-rejectfilter element. The band-reject filter element 400 includes a mainbranch 410, a volume neck 420, and an expansion chamber 450. Theexpansion chamber and the volume next may be filled with an insert 455.In operation, ambient sound can enter an opening 430 of the band-rejectfilter element 400, and certain sound frequencies can be filtered as theambient sound passes through the main branch 410, volume neck 420 andexpansion chamber 450. The resulting filtered sound can exit the opening440 at the opposite end of the band-reject filter element 400.

The following equation can be used to calculate the dimensions neededfor a band-reject filter element to achieve a desired cutoff frequency:

$f_{c} = {\left( \frac{c}{2\pi} \right)\sqrt{\left( \frac{S_{b}}{LV} \right)}}$

-   -   Where:    -   fc=the cutoff frequency of the band-reject filter element 400;    -   c=speed of sound in the insert 455;    -   L=the length of the volume neck 420;    -   S_(b)=area of the volume neck 420; and    -   V=volume of the expansion chamber 450.

As will be described below, a combination of acoustical filter elementscan be used to achieve a target response curve for a particularenvironment and/or activities that protects the ear from certaindamaging or disturbing sound frequencies while allowing other soundfrequencies to reach the ear.

FIG. 5 is a schematic diagram of a passive acoustical filter 500 thatcan be used within an earbud, headphone or other apparatus to protect ahuman's auditory system from hearing damage and/or audio disturbanceassociated with attending musical events. The passive acoustical filter500 includes a double-stage low-pass filter element 510 arranged inparallel with a single stage high-pass filter element 520. Theconfiguration of a double-stage low-pass filter element in parallel witha single stage high-pass filter element will be referred to herein as a“music acoustical filter”. The double-stage low-pass filter element 510consists of two low-pass filter elements in series. The low-pass filterelements include respective expansion chambers 514 connected by a mainbranch 512. Either or both of the expansion chambers may be filled withrespective inserts 516. The high-pass filter element 520 includes a mainbranch 522 and a side branch 524 which may be filled with an insert 526526. In operation, ambient sound can enter the passive acoustical filter500 at one opening 530 and certain sound frequencies can be filtered asthe ambient sound passes through the high-pass filter element 520 andthe dual-stage low-pass filter element 510. The resulting filtered soundcan exit the passive acoustic filter 500 at the opposite opening 540.The filtered sound exiting at 540 may be approximately the sum of thesound passing through the double-stage low-pass filter element 510 andthe sound passing through the high-pass filter element 520.

Exemplary dimensions for the double stage low-pass filter element 510,are set forth in Table 1 below:

TABLE 1 Exemplary dimensions for the double- stage low-pass filterelement 505 Main branch 512 diameter 0.5 mm Expansion chamber 514diameter 2.6 mm Expansion chamber 514 length  14 mmThese dimensions can be derived from the equation for the low-passfilter element set forth above. Further, these dimensions can be usefulfor creating a compact passive acoustic filter that can be inserted intoear buds, headphones or other apparatuses that fit into or outside anear and for achieving an exemplary cutoff frequency for each low-passfilter of approximately 300 Hz. Unless otherwise stated, the term“approximately” means plus-or-minus 20%. In this example, the resonantfrequencies of the two low-pass filter elements are the same. However,in other music filter designs, the resonant frequencies and dimensionsof the two low-pass filter elements may differ.

Further, inserts 516 made from a reticulated material, for example afoam material having a density of 0.5 g/cm³ and bulk modulus 27 kPA, canbe used the expansion chambers 514 of the low-pass filter element 510 toreduce the speed of sound (e.g., to 200 m/s). The foam material or otherreticulated material can optionally be inserted in both expansionchambers 514 of the double stage low-pass filter 510 to reduce the speedof sound and to help keep the dimensions of the music acoustic filtercompact. For the double-stage low-pass filter 510 having the dimensionsdisclosed in Table 1, filling the expansion chambers 512 with foaminserts 516 of these parameters would lower the cutoff frequency fromapproximately 300 Hz to approximately 200 Hz. To achieve a low-passfilter element having a cutoff frequency of 200 Hz or less without theuse of a material to reduce the speed of sound in the expansionchambers, would require the length of diameter of the expansion chamberby 50% from the dimensions disclosed in Table 1. Such a double-stage lowpass filter may be too large to include within an earbud.

Passive acoustical filter 500 also includes a high-pass filter element520 having a main branch 522, and single side branch 524. Exemplarydimensions for the high-pass filter element 520, are set forth in Table2 below:

TABLE 2 Exemplary Dimensions for the high-pass filter element 520 Sidebranch 524 diameter  0.5 mm Side branch 524 length 3.0 Main branch 522diameter 0.55 mmThese dimensions can be derived from the equation for the high-passfilter element set forth above. Further, these dimensions can be usefulfor creating a compact passive acoustic filter that can be inserted intoear buds, headphones or other apparatuses that fit into or outside anear and for achieving an exemplary cutoff frequency of approximately18,200 Hz.

The dimensions described above for the passive acoustical filter 500 arenon-limiting examples, and other dimensions that can achieve similarhearing protection may also be used. Additionally, the dimensionsdescribed above can be tailored to attenuate frequencies associated withspecific genres of music. Further, the frequency response of a musicacoustical filter can be selected to suit the venue of a musical event.For example, the cut-off frequencies of the low-pass filter elements maybe 50 to 3000 Hz, and the cutoff frequency of the high pass filterelement may be 1,000 to 25,000 Hz.

FIG. 6 is a graph of transmission curves relating to a music acousticfilter having the general configuration of the passive acoustic filter500 as shown in FIG. 5. The transmission curves are plotted along an Xaxis that represents sound frequency measured in hertz (Hz) and a Y axisthat represents transmission (inverse of attenuation) of a passiveacoustic filter or filter element measured in decibels (dB). The curve610 represents the transmission of a loss-less double-stage low-passfilter element having the dimensions given in Table 1, including theincorporation of inserts 516 made from a reticulated material in theexpansion chambers 514 of the double-stage low-pass filter elements. Thecurve 620 represents the transmission of a loss-less high-pass filterelement having the dimensions given in Table 2. The curve 630 representsthe combined transmission of the double stage low-pass filter inparallel with the single stage high-pass filter, including frequencyindependent attenuation that occurs in the small diameter main branchesof the filter elements. The curve 640 is the 80 Phon loudness level fromFIG. 1, normalized to 0 dB at 20 Hz. The curve 640 represents the soundpressure level required to produce apparently equal loudness as afunction of frequency. As illustrated in FIG. 6, the performance of thepassive acoustic filter (as summarized by the transmission curve 630)compliments the 80 Phon equal loudness curve 640, in that the filterprovides highest attenuation (lowest transmission) at frequencies wherethe human ear is most sensitive, and lowest attenuation (highesttransmission) for low and high frequencies where the ear is lesssensitive.

FIG. 7 is a schematic diagram of a passive acoustical filter 700 thatcan be used within an ear bud, headphone or other apparatus to protect ahuman's auditory system from hearing damage and/or audio disturbanceassociated with air and train travel. The passive acoustical filter 700includes a single stage high-pass filter element having a main branch710 and a side branch 720. The configuration of a single high-passfilter element will be referred to herein as a “travel acoustic filter”.In operation, ambient sound can enter the passive acoustical filter 700at one opening 730, and certain sound frequencies can be filtered as theambient sound passes through the main branch 710 and side branch 720.The resulting filtered sound can exit the passive acoustical filter 700at the opposite opening 740.

Exemplary dimensions for the passive acoustical filter 700, inaccordance with an embodiment of the disclosed subject matter, are setforth in Table 3 below:

TABLE 3 Exemplary dimensions for the passive acoustical filter 700 Sidebranch 720 diameter 2 mm Side branch 710 length 20 mm  Main branch 710diameter 2 mmThe above disclosed dimensions can be derived from the equation for thehigh-pass filter element set forth above. Further, these dimensions canbe useful for creating a compact filter that can be inserted intoearbuds, headphones or other apparatuses that fit into or outside anear, and for achieving an exemplary cutoff frequency of approximately1850 Hz.

An insert 725, made from a reticulated material, having dimensionsapproximate to the side branch 720 may optionally be placed into theside branch 720 to slow down the speed of sound and reduce the cutofffrequency to approximately 925 Hz.

The dimensions described above for the travel acoustical filter arenon-limiting examples, and other dimensions that can achieve similarhearing protection may also be used.

FIG. 8 is a graph of transmission curves relating to a travel acousticfilter having the general configuration of the passive acoustic filter700 as shown in FIG. 7. The transmission curves are plotted along an Xaxis that represents sound frequency measured in hertz (Hz) and a Y axisthat represents transmission (inverse of attenuation) of a passiveacoustic filter or filter element measured in decibels (dB).Specifically, FIG. 8, includes a curve 810 that represents thetransmission of a loss-less high-pass filter (e.g., the high-pass filtershown in FIG. 7 and defined in Table 3) as a function of frequency. FIG.8 also includes a curve 820 that represents the transmission of arealistic travel acoustic filter including attenuation that occurs inthe narrow diameter main branch of the filter A travel acoustical filtercan be designed to attenuate low and mid-range frequencies, whileallowing high-range frequencies to pass through the filter unchanged orattenuated by a predetermined amount.

FIG. 9 is a schematic diagram of a passive acoustical filter 900 thatcan be used to protect from hearing damage and/or audio disturbanceassociated with attending a sports event or participating in motor sportactivities (e.g., jet skiing, wave running, motorcycling). The passiveacoustical filter 900 includes a single stage low-pass filter element910 and a single stage high-pass filter element 920 in parallel. Theconfiguration of a single stage low-pass filter element and a singlestage high-pass filter element in parallel will be referred to herein asa “sports filter”. The low-pass filter element 910 includes a mainbranch 912 and an expansion chamber 914. The expansion chamber mayoptionally be filled with a medium 916. The high-pass filter elementincludes a main branch 922 and a side branch 924. In operation, ambientsound can enter the passive acoustical filter 900 at one opening 930,and certain sound frequencies can be filtered as the ambient soundpasses through the low-pass filter element 910 and the high-pass filterelement 920. The filtered sound can exit the passive acoustical filter900 at the opposite opening 940.

Exemplary dimensions for the low-pass filter element 910 are set forthin Table 4 below:

TABLE 4 Exemplary Dimensions for the low-pass filter element 910 Mainbranch 912 diameter 0.8 mm Expansion chamber 914 diameter 5 mm Expansionchamber 914 length 12 mmThe disclosed dimensions can be derived from the equation for thelow-pass filter set forth above. Further, these dimensions can be usefulfor creating a compact sports filter that can be inserted into ear buds,headphones or other apparatuses that fit into or outside an ear, and forachieving an exemplary cutoff frequency for the low-pass filter elementof approximately 240 Hz.

Further, a insert 916 of reticulated material having dimensionsapproximate to the expansion chamber 914 can optionally be placed in theexpansion chamber of the low-pass filter 910 to reduce the speed ofsound and thus lower the cut-off frequency of the low-pass filterelement 910 to approximately 160 Hz.

Exemplary dimensions for the high-pass filter element 920 are set forthin Table 5 below:

TABLE 5 Exemplary dimensions for the high-pass filter element 920 Sidebranch 924 diameter 0.8 mm Side branch 924 length  12 mm Main branch 922diameter 0.8 mmThe disclosed dimensions can be derived from the equation for thehigh-pass filter set forth above. Further, these dimensions can beuseful for creating a compact sports filter that can be inserted intoearbuds, headphones or other apparatuses that fit into or outside anear, and for achieving an exemplary cutoff frequency for the high-passfilter element of approximately 4.5 kHz.

The dimensions described above for the low-pass filter element 910 andthe high-pass filter element 920 of the passive acoustical filter 900are non-limiting examples, and other dimensions that can achieve similarhearing protection for sports events and motor activities may also beused. Additionally, the dimensions described above can be tailored forindoor versus outdoor sports stadium use. Because indoor stadiumsgenerate greater midrange sound frequencies than outdoor stadiums, anindoor sports acoustical filter may be designed to attenuate moremidrange frequencies (e.g., the midrange sound frequencies generated bypeoples' voices) relative to the bass and treble sounds (e.g., from feetstomping, sound systems and fireworks). For example, the cut-offfrequency of the low-pass filter element may be 50 to 3000 Hz, and thecutoff frequency of the high pass filter element may be 1,000 to 20,000Hz.

FIG. 10 is a graph of transmission curves relating to a sports acousticfilter having the general configuration of the passive acoustic filter900 as shown in FIG. 9. Like FIG. 6 and FIG. 8, the transmission curvesare plotted along an X axis that represents sound frequency measured inhertz (Hz) and a Y axis that represents transmission (inverse ofattenuation) of a passive acoustic filter or filter element measured indecibels (dB). The curve 1010 represents the transmission of a loss-lesslow-pass filter element having the dimensions given in Table 4 and theexpansion chamber filled with a medium to lower the cut-off frequency toapproximately 160 Hz. The curve 1020 represents the transmission of aloss-less high-pass filter element having the dimensions given in Table5. The curve 1030 represents the combined transmission of the doublestage low-pass filter in parallel with the single stage high-passfilter, including attenuation that occurs in the narrow diameter mainbranches of the low-pass and high-pass filter elements. The curve 1040is the 80 Phon loudness level from FIG. 1, normalized to 0 dB at 20 Hz.The curve 1040 represents the sound pressure level required to produceapparently equal loudness as a function of frequency. As illustrated inFIG. 10, the performance of the sports acoustic filter (as summarized bythe transmission curve 1030) compliments the 80 Phon equal loudnesscurve 1040, in that the filter provides highest attenuation (lowesttransmission) at frequencies where the human ear is most sensitive, andlowest attenuation (highest transmission) for low and high frequencieswhere the ear is less sensitive.

FIG. 11 is a schematic diagram of a passive acoustical filter 1100 thatcan be used within earbuds, headphones or other apparatuses to protect ahuman's auditory system from auditory disturbance during sleep.Acoustical filter 1100 includes a double stage low-pass filter 1110including two low-pass filter elements in series. A filter configurationconsisting of two low-pass filter elements in series will be referred toherein as a “sleep filter”. The low pass filter elements have respectiveexpansion chambers 1114 connected by a main branch 1112. Either or bothof the expansion chambers 1114 may be filled inserts 1116 made from areticulated material. In operation, ambient sound can enter the passiveacoustical filter 1100 at one opening 1130, and certain soundfrequencies can be filtered as the ambient sound passes through the mainbranch 11112 and two expansion chambers 1114. The filtered sound canexit the passive acoustical filter 1100 at the opposite opening 1140.

Exemplary dimensions for the sleep filter, in accordance with anembodiment of the disclosed subject matter, are set forth in the tablebelow:

TABLE 6 Exemplary Dimensions for the passive acoustical filter 1100 Mainbranch 1112 diameter 0.5 mm Expansion chamber 1114 diameter 4 mmExpansion chamber 1114 length 13 mmThese dimensions can be derived from the equation for the low-passfilter set forth above. Further, these dimensions can be useful forcreating a compact filter that can be inserted into earbuds, headphonesor other apparatuses that fit into or outside an ear, and for achievingan exemplary cutoff frequency for the low-pass filter of approximately130 Hz.

Further, inserts 1116 made from a reticulated material (e.g., foam)having dimensions approximate to the expansion chambers 1114 canoptionally be inserted into the expansion chambers 1114 of the doublestage low-pass filter 1110 to reduce the speed of sound and lower thecutoff frequency to 90 Hz.

The dimensions described above for the double stage low-pass filter ofthe sleep filter are non-limiting examples, and other dimensions thatcan achieve similar hearing protection from sleep disturbances may alsobe used. For example, the cut-off frequency of each low-pass filterelement may be 50 to 3000 Hz.

FIG. 12 is a graph of transmission curves relating to a sleep acousticfilter having the general configuration of the passive acoustic filter1100 as shown in FIG. 11. The transmission curves are plotted along an Xaxis that represents sound frequency measured in hertz (Hz) and a Y axisthat represents transmission (inverse of attenuation) of a passiveacoustic filter or filter element measured in decibels (dB).Specifically, FIG. 12 includes a curve 1210 that represents thetransmission of an exemplary loss-less double-stage low-pass filter(e.g., the double stage low-pass filter shown in FIG. 11 and defined inTable 6) as a function of frequency. FIG. 12 also includes a curve 1220that represents the transmission of the exemplary double-stage low-passfilter including attenuation that occurs in the narrow main branch ofthe filter and the use of the reticulated inserts to fill the expansionchambers. A sleep acoustical filter can be designed to attenuate mid-and high-range frequencies, while allowing low-range frequencies to passthrough the filter unchanged or with a predetermined attenuation.

Other passive acoustical filters may include a band-reject filterelement to attenuate frequencies at a specific frequency range. Thedimensions of the band-reject filter element can be tailored to filterout sound at a targeted frequency range on the sound frequency spectrum.For example, a band-reject filter element, similar to the example shownin FIG. 4, can be used to filter our sound frequencies associated with aconsistent whine or hum (e.g., power generation plant), while allowingother frequencies to pass through.

Other passive acoustical filters may include a band-pass filter elementcan be used to allow only a specific range of sound frequencies to passthrough unchanged, while attenuating sounds at all other frequencies. Aband-pass filter element may be realized, for example, by placing alow-pass filter element and a high-pass filter element in series, withthe cutoff frequency of the high-pass filter element set lower than thecutoff frequency of the low-pass filter element.

Passive acoustical filters, such as those described above, may bedisposed within a housing that can be substantially contained within theear (e.g., in the ear canal, concha, and/or pinna. Passive acousticalfilters, such as those described above, may be disposed outside the earin a headphone apparatus or any other apparatus that allows ambientsound to travel through an acoustical filter.

FIG. 13A is a graphical image of components of an exemplary musicalacoustical filter 1300. The musical acoustic filter 1300 includeshousing 1310 made of a plastic or metal material, a center divider 1320,two inserts 1330 made of foam or other reticulated material, and firstand second end caps 1340, 1350. In this example, the housing 1310 is acylindrical tube. The housing 1310 may have a cross-sectional shapeother than cylindrical, such as square or hexagonal. The reticulatedinserts 1330 may include an adhesive backing for attaching to the centerdivider 1320. The end caps 1340, 1350 incorporate apertures for theinput and output of the music acoustical filter 1300 and channels thatconnect the other elements of the music acoustical filter 1300. In thiscontext, the term “channels” means any passage through which acousticwaves may pass. Channels may commonly be cylindrical passages or tubes.As shown in the cross-sectional schematic view of FIG. 13B, the centerdivider 1320 together with the walls of the outer tube 1310 form twoexpansion chambers 1360 that are substantially filled by the reticulatedinserts 1330. Additionally, first and second longitudinal channels 1322,1324 run through the length the center divider 1320.

FIG. 14 is a schematic diagram of the music acoustical filter 1300. Thecentral portion of the device (comprising the outer tube 1310, centerdivider 1320, and reticulated inserts 1330, as shown in FIG. 13A)includes two expansions chambers 1360 that are substantially filled byreticulated inserts 1330. The two expansion chambers are connected inseries by the first longitudinal channel 1322 (running the length of thecenter divider 1320) and channels 1412 (formed in the end caps 1340,1350) to form a two-stage low pass filter. The second longitudinalchannel 1324 (running the length of the center divider 1320) forms themain branch of a high-pass filter element, which is connected inparallel to the two-stage low-pass filter element by additional channels1412 in the end caps 1340, 1350. One of the end caps (end cap 1340 asshown in FIG. 14) includes a side branch 1424 of the high-pass filterelement. Ambient sound can enter the music acoustical filter 1300through an aperture 1430 in end cap 1340 and certain sound frequenciescan be filtered as the ambient sound passes through the high-pass filterelement and the dual-stage low-pass filter element. The resultingfiltered sound can exit the music acoustical filter 1300 through anaperture 1440 in end cap 1350. The filtered sound exiting at 1440 may beapproximately the sum of the sound passing through the double-stagelow-pass filter element and the sound passing through the high-passfilter element. The dimensions of the music acoustical filter 1300 maybe selected to provide the desired filter function, as previouslydiscussed with respect to the music acoustical filter 500 of FIG. 5.FIG. 15 is a graphical image of the exemplary music acoustical filter1300 assembled from the components shown in FIG. 13A. The outer tube1310 and end caps 1340, 1350 are visible.

FIG. 16 shows an exploded view of another exemplary music acousticalfilter 1600. The exploded music acoustical filter 1600 includes a doublestage low-pass filter formed by a first body piece 1610 and a secondbody piece 1620. The first and second body pieces 1610, 1620 may formedfrom a plastic material such as polycarbonate. Each of the first bodypiece 1610 and the second body piece 1620 includes a symmetric half of adouble-stage low-pass filter element 1630 in the shape of an “S.” Firstbody piece 1610 includes half-cylindrical recesses forming two expansionchambers 1632 and connecting channels 1634, 1636, 1638. Second bodypiece 1620 includes mirror images recesses (not visible). Whenassembled, the first body piece 1610 and the second body piece 1620collectively form two cylindrical expansion chambers connected in seriesby cylindrical main channels.

The second body piece 1620 also includes a cavity 1622 for placement ofan insert 1640 which may be formed from polycarbonate or anotherplastic. When the insert 1640 is placed into the cavity 1622, aninterstitial space between the insert 1640 and the second body piece1620 forms a main branch of a high-pass filter element. A side branch ofthe high-pass filter element is formed by the two holes 1642. Two holesare used in this example to provide sufficient cross-sectional area forthe side branch. A passage 1624 in the body piece 1620 may couple thehigh-pass filter element to the channel 1638. A groove 1644 in theinsert 1640 provides an outlet to the ambient for the side branch of thehigh-pass filter element.

An end cap 1650 may fit over the end portions of first and second bodypieces 1610 and 1620 to hold the pieces in alignment. The end cap mayoptionally include a scrim cloth designed to protect the filter from earsurface contaminants.

FIG. 17 shows the exemplary music filter 1600 assembled from thecomponents shown in FIG. 16. FIG. 17 shows the first body piece 1610,the second body piece 1620, the inset 1640, and the end cap 1650.

FIG. 18 shows a transparent view of an exemplary earbud 1800 containinga passive acoustical filter 1810, which may be, for example, the musicacoustic filter 1600 (as shown in FIG. 17) or the music acousticalfilter 1300 (as shown in FIG. 15). The music acoustical filter 1810 isshown inserted into the neck of the exemplary earbud 1800. An outerremovable cap 1815 may be positioned on top of the filter and can beinserted into the ear. The exemplary earbud 1800 may be, for example,the earbud described in Design patent application 29/485,359.

Passive acoustical filters as described above can be designed to beinterchangeable, so they can each fit into the same sized earbud,headphone or other apparatus. Designing various passive acoustic filtersfor interchangeability can allow for the different types of acousticfilters to be swapped in and out of the earbud, headphone or otherapparatus, in order to match the type of auditory protection desired fordifferent environments and/or activities.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of elements, it should beunderstood that those elements may be combined in other ways toaccomplish the same objectives. Elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A passive acoustical filter, comprising: a firstopening to receive ambient sound; a second opening to output filteredsound; a first low pass filter element coupled between the first openingand the second opening, the first low pass filter element comprising afirst expansion chamber filled with a material in which a speed of soundis lower than a speed of sound in air.
 2. The passive acoustical filterof claim 1, wherein the material in which the speed of sound is lowerthan the speed of sound in air is a reticulated material.
 3. The passiveacoustical filter of claim 2, wherein the reticulated material isselected from a plastic foam material, cotton fibers, bamboo fibers, andyarn.
 4. The passive acoustical filter of claim 1, wherein a cutofffrequency of the first low pass filter element is less than or equal to200 Hertz.
 5. The passive acoustical filter of claim 1, furthercomprising: a high pass filter element coupled between the first openingand the second opening in parallel with the first low pass filterelement.
 6. The passive acoustical filter of claim 1, wherein the firstlow pass filter element is part of a dual-stage low pass filter elementcoupled between the first opening and the second opening, the dual-stagelow pass filter element comprising the first low pass filter element anda second low pass filter element in series, the second low pass filtercomprising a second expansion chamber.
 7. The passive acoustical filterof claim 6, wherein the second expansion chamber is filled with thematerial in which a speed of sound is lower than a speed of sound inair.
 8. The passive acoustical filter of claim 6, further comprising: ahigh pass filter element coupled between the first opening and thesecond opening in parallel with the dual-stage low pass filter element.9. A passive acoustical filter comprising: an earbud including a firstopening to receive sound from an ambient and a second opening to outputfiltered sound to an ear of a listener; and a first-low-pass filterelement contained within the earbud and coupled between the firstopening and the second opening, the low-pass filter element comprisingan expansion chamber filled with a material in which a speed of sound islower than a speed of sound in air.
 10. The passive acoustical filter ofclaim 9, wherein the material in which the speed of sound is lower thanthe speed of sound in air is a reticulated material.
 11. The passiveacoustical filter of claim 10, wherein the reticulated material isselected from a plastic foam material, cotton fibers, bamboo fibers, oryarn.
 12. The passive acoustical filter of claim 9, wherein a cutofffrequency of the first low pass filter element is less than or equal to200 Hertz.
 13. The passive acoustical filter of claim 9, furthercomprising: a high pass filter element contained within the earbud andcoupled between the first opening and the second opening in parallelwith the first low pass filter element.
 14. The passive acousticalfilter of claim 9, wherein the first low pass filter element is part ofa dual-stage low pass filter element coupled between the first openingand the second opening, the dual-stage low pass filter elementcomprising the first low pass filter element and a second low passfilter element in series, the second low pass filter comprising a secondexpansion chamber.
 15. The passive acoustical filter of claim 14,wherein the second expansion chamber is filled with the material inwhich a speed of sound is lower than a speed of sound in air.
 16. Thepassive acoustical filter of claim 14, further comprising: a high passfilter element contained within the earbud and coupled between the firstopening and the second opening in parallel with the dual-stage low passfilter element.